1. Field of the Invention
The present invention relates to a light emitting device in which a light emitting element and a transistor for controlling the light emitting element are provided on a semiconductor substrate or an insulating surface, and to a method of driving the light emitting device. More specifically, the invention relates to a light emitting device and method of driving the same in which influence of fluctuation in characteristic of transistors which control light emitting elements is removed. The present invention belongs to a technical field related to a light emitting device using a semiconductor element such as a transistor.
2. Description of the Related Art
In recent years, development of light emitting devices using light emitting elements (image display devices) is being advanced. Light emitting devices are roughly divided into passive type and active type. Active light emitting devices each have a light emitting element and a transistor for controlling the light emitting element on an insulating surface.
Transistors using polysilicon films are higher in field effect mobility (also called mobility) than conventional transistors that are formed of amorphous silicon films, and therefore can operate at higher speed than the transistors formed of amorphous silicon films. For that reason, control of pixels, which has conventionally been carried out by a driving circuit external to the substrate, can be conducted by a driving circuit formed on the same insulating surface where the pixels are formed. Such active light emitting devices obtain various advantages including reduction in production cost, reduction in size, a rise in yield, and improvement of throughput by building various kinds of circuits and elements on the same insulating surface.
Major driving methods of active light emitting devices are analog methods and digital methods. The former methods, namely, the analog methods control a current flowing into a light emitting element to control the luminance and obtain gray scale. On the other hand, the latter methods, namely, the digital methods drive the devices by switching between only two states, ON state in which a light emitting element is ON (the luminance thereof is almost 100%) and OFF state in which the light emitting element is OFF (the luminance thereof is almost 0%). This allows only two gray scales and, therefore, techniques for obtaining multi-gray scale by combining this with a time gray scale method, an area ratio gray scale, or the like have been proposed for the digital methods.
Now, a detailed description will be given with reference to FIG. 14 and FIGS. 15A and 15B on a method of driving a light emitting device. The structure of the light emitting device is described first referring to FIG. 14. FIG. 14 shows an example of circuit diagram of a pixel portion 1800 in the light emitting device. Gate signal lines (G1 to Gy), which transmit gate signals supplied from a gate signal line driving circuit to pixels, are connected to gate electrodes of switching transistors. The switching transistors are provided in the respective pixels and each denoted by 1801. The switching transistor 1801 of each pixel has a source region and a drain region one of which is connected to one of source signal lines (S1 to Sx) for inputting video signals and the other of which is connected to a gate electrode of a driving transistor 1804 of each pixel and to a capacitor 1808 of each pixel.
The driving transistor 1804 of each pixel has a source region connected to one of power supply lines (V1 to Vx) and has a drain region connected to a light emitting element 1806. The electric potential of the power supply lines (V1 to Vx) is called a power supply electric potential. Each of the power supply lines (V1 to Vx) is connected to the capacitor 1808 of each pixel.
The light emitting element 1806 has an anode, a cathode, and an organic compound layer interposed between the anode and the cathode. If the anode of the light emitting element 1806 is connected to the drain region of the driving transistor 1804, the anode serves as a pixel electrode while the cathode of the light emitting element 1806 serves as an opposite electrode. On the other hand, if the cathode of the light emitting element 1806 is connected to the drain region of the driving transistor 1804, the anode of the light emitting element 1806 serves as the opposite electrode whereas the cathode serves as the pixel electrode.
The electric potential of the opposite electrode is called an opposite electric potential and a power supply that gives the opposite electric potential to the opposite electrode is called an opposite power supply. The difference between the electric potential of the pixel electrode and the electric potential of the opposite electrode is a drive voltage, and the drive voltage is applied to the organic compound layer.
FIGS. 15A and 15B are timing charts for when the light emitting device of FIG. 14 is driven by an analog method. In FIGS. 15A and 15B, a period starting with selection of one gate signal line and ending with selection of the next gate signal line is called one line period (L). A period started as one image is displayed and ended as the next image is displayed is called one frame period (F). The light emitting device of FIG. 14 has y gate signal lines and therefore y line periods (L1 to Ly) are provided in one frame period.
The power supply lines (V1 to Vx) are held at a constant power supply electric potential. The opposite electric potential that is the electric potential of the opposite electrode is also kept constant. The opposite electric potential is set such that the difference between it and the power supply electric potential is large enough to cause the light emitting element to emit light.
In the first line period (L1), the gate signal line (G1) is selected by a gate signal supplied from the gate signal line driving circuit. A gate signal line being selected means that a transistor whose gate electrode is connected to the gate signal line is turned ON.
Then analog video signals are inputted sequentially to the source signal lines (S1 to Sx). Since every switching transistor 1801 that is connected to the gate signal line (G1) is turned ON, the video signals inputted to the source signal lines (S1 to Sx) are inputted to the gate electrode of the driving transistor 1804 through the switching transistor 1801.
The amount of current flowing in a channel formation region of the driving transistor 1804 is controlled by the level of electric potential (voltage) of a signal inputted to the gate electrode of the driving transistor 1804. Therefore, the level of electric potential applied to the pixel electrode of the light emitting element 1806 is determined by the level of electric potential of the video signal inputted to the gate electrode of the driving transistor 1804. In short, a current flow in the light emitting element 1806 in an amount according to the level of electric potential of a video signal and the light emitting element 1806 emits light in accordance with this current amount.
The operation described above is repeated until inputting video signals to the source signal lines (S1 to Sx) is completed. This is the end of the first line period (L1). Then the second line period (L2) is started and the gate signal line (G2) is selected by a gate signal. Similar to the first line period (L1), video signals are sequentially inputted to the source signal lines (S1 to Sx).
The above operation is repeated until inputting gate signals to all the gate signal lines (G1 to Gy) is completed, thereby ending one frame period. During one frame period, all pixels are used to form an image for display.
As has been described, a method which uses a video signal to control the amount of current flowing into a light emitting element and in which the gray scale is determined in accordance with the current amount is a driving method called an analog type. In short, the gray scale is determined in accordance with the electric potential of a video signal inputted to a pixel in the analog driving method.
On the other hand, in a digital driving method, multi-gray scale is obtained in combination with a time gray scale method or the like as described above. In a digital driving method combined with a time gray scale method, the gray scale is determined in accordance with the length of a period in which a current flows between two electrodes of a light emitting element (a detailed timing chart of this is not provided).
Described next with reference to FIGS. 11A to 13 is voltage-current characteristics of the driving transistor 1804 and light emitting element 1806. FIG. 11A shows the driving transistor 1804 and the light emitting element 1806 alone out of the pixel shown in FIG. 14. FIG. 11B shows voltage-current characteristics of the driving transistor 1804 and light emitting element 1806 of FIG. 11A. The voltage-current characteristic graph of the driving transistor 1804 in FIG. 11B shows the amount of current flowing in the drain region of the driving transistor 1804 in relation to a voltage VDS between the source region and the drain region. FIG. 12 shows plural voltage-current characteristic curves different from each other in VGS that is a voltage between the source region and gate electrode of the driving transistor 1804.
As shown in FIG. 11A, a voltage applied between the pixel electrode and opposite electrode of the light emitting element 1806 is given as VEL, and a voltage applied between a terminal 3601 that is connected to the power supply line and the opposite electrode of the light emitting element 1806 is given as VT. The value of VT is fixed by the electric potential of the power supply lines (V1 to Vx). VDS represents a voltage between the source region and drain region of the driving transistor 1804, and VGS represents a voltage between a wire 3602 connected to the gate electrode of the driving transistor 1804 and the source region, namely, a voltage between the gate electrode and source region of the driving transistor 1804.
The driving transistor 1804 and the light emitting element 1806 are connected to each other in series. This means that the same amount of current flows in the elements (the driving transistor 1804 and the light emitting element 1806). Therefore the driving transistor 1804 and light emitting element 1806 shown in FIG. 11A are driven at intersections (operation points) of the curves that indicate the voltage-current characteristics of the elements. In FIG. 11B, VEL corresponds to a voltage between the electric potential of the opposite electrode 1809 and the electric potential at the operation point. VDS corresponds to a voltage between the electric potential of the driving transistor 1804 at the terminal 3601 and the electric potential of 1804 at the operation point. Accordingly, VT is equal to the sum of VEL and VDS.
Here, consider a case in which VGS is changed. As can be seen in FIG. 11B, the amount of current flowing into the driving transistor 1804 is increased as |VGS-VTH| of the driving transistor 1804 is increased, in other words, as |VGS| is increased. VTH represents the threshold voltage of the driving transistor 1804. Therefore, as FIG. 11B shows, a rise in |VGS| is naturally followed by an increase in amount of current flowing in the light emitting element 1806 at an operation point. The luminance of the light emitting element 1806 is raised in proportion to the amount of current flowing in the light emitting element 1806.
When the amount of current flowing in the light emitting element 1806 is increased accompanying a rise in |VGS|, VEL is accordingly increased. When VEL is increased, VDS is reduced that much since VT is a fixed value determined by the electric potential of the power supply lines (V1 to Vx).
As shown in FIG. 11B, a voltage-current characteristic curve of the driving transistor 1804 can be divided into two ranges by the values of VGS and VDS. A range in which |VGS-VTH|<|VDS| is a saturation range, and a range in which |VGS-VTH|>|VDS| is a linear range.
In the saturation range, the following expression (1) is satisfied. IDS is given as the amount of current flowing in the channel formation region of the driving transistor 1804. β=μCoW/L, wherein μ represents the mobility of the driving transistor 1804, Co represents the gate capacitance per unit area, and W/L represents the ratio of a channel width W of the channel formation region to its channel length L.
[Mathematical Expression 1]IDS=β(VGS−VTH)2  (1)
In the linear range, the following expression (2) is satisfied.
[Mathematical Expression 2]|IDS=β{(VGS−VTH)VDS−VDS2}  (2)
It is understood from the expression (1) that the current amount in the saturation range is hardly changed by VDS but is determined solely by VGS.
It is understood from the expression (2) that the current amount in the linear range is determined by VDS and VGS. As |VGS| is increased, the driving transistor 1804 comes to operate in the linear range. VEL is also increased gradually. Accordingly, VDS is reduced as much as VEL is increased. When VDS is reduced, the current amount is also reduced in the linear range. For that reason, the current amount is not easily increased despite an increase in |VGS|. The current amount reaches IMAX when |VGS|=∞. In other words, a current larger than IMAX does not flow no matter how large |VGS| is. IMAX represents the amount of current flowing in the light emitting element 1806 when VEL=VT.
By controlling the level of |VGS| in this way, the operation point can be moved to the saturation range, or to the linear range.
Ideally, every driving transistor 1804 has the same characteristic. However, in reality, the threshold voltage VTH and the mobility μ often vary from one driving transistor 1804 to another. When the threshold voltage VTH and the mobility μ vary from one driving transistor 1804 to another, as the expressions (1) and (2) show, the amount of current flowing in the channel formation region of the driving transistor 1804 fluctuates even though VGS is the same.
FIG. 12 shows the voltage-current characteristic of the driving transistor 1804 whose threshold voltage VTH and mobility p are deviated from ideal ones. A solid line 3701 indicates the ideal voltage-current characteristic curve. 3702 and 3703 each indicate the voltage-current characteristic of the driving transistor 1804 whose threshold VTH and mobility μ differ from ideal ones.
The voltage-current characteristic curves 3702 and 3703 in the saturation range deviate from the ideal current-voltage characteristic curve 3701 by the same current amount ΔIA. An operation point 3705 of the voltage-current characteristic curve 3702 is in the saturation range whereas an operation point 3706 of the voltage-current characteristic curve 3703 is in the linear range. In this case, the current amount at the operation point 3705 and the current amount at the operation point 3706 are shifted from the current amount at an operation point 3704 of the ideal voltage-current characteristic curve 3701 by ΔIB and ΔIC, respectively. ΔIC at the operation point 3706 in the linear range is smaller than ΔIB at the operation point 3705 in the saturation range.
To conclude the above operation analysis, a graph of current amount in relation to the gate voltage |VGS| of the driving transistor 1804 is shown in FIG. 13. When |VGS| is increased until it exceeds the absolute value of the threshold voltage of the driving transistor 1804, namely, |VTH|, the driving transistor 1804 is turned conductive and a current starts to flow. If |VGS| is further increased, |VGS| reaches a value that satisfies |VGS-VTH|=|VDS| (here, the value is denoted by A) and the curve leaves the saturation range to enter the linear range. If |VGS| is increased still further, the current amount increases and finally reaches saturation. At this point, |VGS|=∞.
As can be understood from FIG. 13, almost no current flows in a range where |VGS|≦VTH|. A range in which |VTH|≦|VGS|≦A is satisfied is called a saturation range and the current amount is changed by |VGS| in this range. This means that, if the voltage applied to the light emitting element 1806 in the saturation range is changed even slightly, the amount of current flowing in the light emitting element 1806 is changed exponentially. The luminance of the light emitting element 1806 is raised almost in proportion to the amount of current flowing in the light emitting element 1806. To summarize, the device mainly operates in the saturation range in an analog driving method that controls the amount of current flowing into the light emitting element in accordance with |VGS| to control the luminance and obtain gray scale.
On the other hand, a range where A≦|VGS| in FIG. 13 is the linear range and the amount of current flowing into the light emitting element is changed by |VGS| and |VDS| in this range. In the linear range, the amount of current flowing in the light emitting element 1806 is not changed much when the level of voltage applied to the light emitting element 1806 is changed. A digital driving method drives the device by switching between only two states, ON state in which the light emitting element is ON (the luminance thereof is almost 100%) and OFF state in which the light emitting element is OFF (the luminance thereof is almost 0%). When the device operates in the range where A≦|VGS| in order to turn the light emitting element ON, the current value approaches IMAX without fail and the luminance of the light emitting element reaches almost 100%. On the other hand, when the device operates in the range where |VTH≧|VGS| in order to turn the light emitting element OFF, the current value is almost 0 and the luminance of the light emitting element reaches almost 0%. In short, a light emitting device driven by a digital method mainly operates in ranges where |VTH|≧|VGS| and A≦|VGS|.
In a light emitting device driven by an analog method, when a switching transistor is turned ON, an analog video signal inputted to a pixel turns into a gate voltage of a driving transistor. At this point, the electric potential of a drain region of the driving transistor is determined in accordance with the voltage of the analog video signal inputted to a gate electrode of the driving transistor and a given drain current flows into a light emitting element. The light emitting element emits light in an amount (at a luminance) according to the drain current amount. The light emission amount of a light emitting element is controlled as described above, thereby obtaining gray scale display.
However, the analog method described above has such a drawback that it is very weak against fluctuation in characteristic among driving transistors. With driving transistors of the respective pixels fluctuated in characteristic, it is impossible to supply the same amount of drain current even when the same level of gate voltage is applied to the driving transistors. In other words, the slightest fluctuation in characteristic among driving transistors causes light emitting elements to emit light in greatly varying amount even though the light emitting elements receive a video signal of the same voltage level.
Analog driving methods are thus responsive to fluctuation in characteristic among driving transistors and it has been a liability in gray scale display by conventional active light emitting devices.
If a light emitting device is driven by a digital method in order to deal with fluctuation in characteristic among driving transistors, the amount of current flowing into an organic compound layer of a light emitting element is changed accompanying degradation of the organic compound layer.
This is because light emitting elements are degraded with age by nature. Voltage-current characteristic curves of a light emitting element before and after degradation are shown in the graph of FIG. 18A. In a digital driving method, a light emitting device operates in a linear range as described above. When a light emitting element is degraded, its voltage-current characteristic curve is changed as shown in FIG. 18A to shift its operation point. This causes a change in amount of current flowing between two electrodes of the light emitting element.